Mason-Pfizer Monkey virus (M-PMV) is an exogenous, primate retrovirus that was isolated from a breast carcinoma of a rhesus monkey. It is the prototype virus of the D-type retroviruses which, like the B type retrovirus mouse mammary tumor virus, preassemble a complete capsid within the cytoplasm of an infected cell. These A-type particles migrate to the plasma membrane and are enveloped by a lipid bilayer upon release from the cell. The major goals of this proposal are to define those viral coded gag-gene functions that are essential in assembly and morphogenesis of the M-PMV particle. The amino terminal and carboxy-terminal amino acid sequence of the five major gag gene products of this virus will be determined. This information will be used to locate the polypeptide coding region on the nucleic acid sequence of the gag gene determined from cloned M-PMV DNA. In vitro site-directed mutagenesis of the gag gene coding region and of a sequence 3' of gag which may code for the viral protease will be used to determine the function of these regions.